If you could do as the node title suggests, I would go so far as to suggest that you wouldn't even be speaking Japanese. Indeed, the Japanese language has no singular "r", or the associated /r/ /l/ /d/ aveolarstop, which I will refer to as the symbol "r" for the sake of simplicity. The Japanese language does not contain singular consonants, with the exception of the so-called syllabic n. All of the other syllables contain vowels.

Most Japanese speakers are incapable of separating the /r/ consonant phoneme from the vowels of ra/ri/ru/re/ro, the seventh row ("ra-yuki") of the gojuuon (Japanese "phonetic alphabet"). Thus, while they can easily say the syllables "ra", "ri", "ru", "re", and "ro", as well as the associated dipthongs "rya", "ryu", and "ryo", saying just the "r" is both unexpectedly difficult and completely useless for them.

Gritchka also points out that, as a stop, the Japanese /r/ is impossible to atriculate on its own. The English /r/ is a liquid, and can be prolonged and easily articulated by itself, as in the word "Grrrrrrr!!".

There is no real English analogue to this, but consider the following. Say just the very first part of an "r", but then finish with the middle of "d". This is a rough description of how to say the consonant component that this node focuses on, mapped to English phonemes. That's probably about how hard it is for a Japanese speaker to logically separate "r" from "ra".

So how am I supposed to say the R then!

Most Japanese teachers will recommend that you practice ra/ri/ru/re/ro instead of the R sound itself.

The initial "r" and the vowel "ae" are not too hard for the Japanese student to closely approximate with the Japanese "re" syllable.

However, the closing "r" is nearly impossible -- lacking a liquid "r" phoneme, the student is left with little choice but the enlongated "a" vowel.

This gives a resulting two-syllable word that sounds like "rayah" or "layah". (The "y" sound is again perceived by the English listener, as in yen.) Actually, this word exists in Japanese as gairaigo to mean uncooked, from the English expression "rare steak".

"rya", "ryu", and "ryo" dipthongs

These dipthongs are among the hardest Japanese phonemes for English speakers to master, both in speaking and listening. Again, this is because they move through several English phonemes in a single movement. The only real course of action is to first master the "ri" syllable and then work on combining it with "ya".

Like all of the ra-yuki, these are aveolar tapped sounds, ie., your tongue should make brief contact with the ridge in the top of your mouth at the start of the sound.

rolling?

The trilled / rolling "r" that esteemed noders Juuichiketajin and tongpoo describe above is also seen when Japanese speakers (esp. men) are extremely angry -- for example, the interjection "kora!" (an angry attention-getter) can sound like KORRRAA! This rolling is also seen in stereotypical depictions of yakuza on TV, as in the fundamental expression "kono yarou!"